The present invention relates generally to power control circuits and specifically concerns a circuit for controlling the power delivered to a universal direct current motor.
Many driven devices including universal direct current motors currently available on the market incorporate a very inexpensive design typically characterized by a mininum number of magnetic poles, high rotational friction and mechanical binding due to gear ratio-ing mechanisms. Due to these inherent design deficiencies most such driven devices have a relatively poor low voltage response and therefore will not operate smoothly at low speeds. In addition to the operational limitations imposed on such driven devices as a result of the aforementioned inherent design deficiencies, the control apparatus used to deliver power to the device can also influence its operation for the better or worse.
A typical prior art control apparatus for delivering power to a universal direct current motor comprises a bridge rectifier or the like for providing an unfiltered half or full-wave rectified signal whose amplitude is controlled by a variable resistance to increase or decrease the response of the driven device. U.S. Pat. No. 4,202,032, for example, discloses a power controller wherein the half and full-wave rectified signals are combined prior to being applied to the driven device. Power control circuits of this type, although inexpensive to manufacture and while providing for better operation than a controlled pure direct current voltage, are nevertheless unsatisfactory for insuring smooth performance of the driven device.
Power control circuits of the foregoing type typically include a variable resistance in the form of a rheostat to minimize power dissipation. Consequently, there is a point near the minimum operational range when the circuit is "open" thereby limiting the range of control over the driven device. Slow speed performance in response to such power control circuits is further aggravated by the resistance of the control circuit relative to that of the driven device.
Attempts to improve upon the performance of such prior art power control circuits have included the use of transistors as current amplifiers to allow for full range operation. However, the high power dissipation associated with these circuits requires the use of relatively large heat sinks to prevent damage to the transistors. Also, the voltage loss across the transistor, typically on the order of 1.5 to 2.5 volts, can be a relatively large percentage of the total voltage available to the control circuit.
Other circuits have been developed which rely on phase techniques for controlling the power delivered to the driven device. These circuits overcome some of the heat dissipation problems associated with the transistor control circuits but other problems are encountered. In particular, the phase control is typically implemented by silicon controlled rectifiers which supply extremely fast risetime pulses to the driven device resulting in excessive heating and premature failure of the driven device. It is known to connect capacitors across the output of the control circuit in order to reduce the risetime of the output pulses, but such capacitors also have the tendency to degrade the phase control operation of the circuit. Further, the voltage drop across the silicon controlled rectifier may be in excess of 1.2 volts which is still a relatively large percentage of the control circuit supply voltage.
It is accordingly a general object of the present invention to provide an improved power control circuit for controlling the power delivered to a driven device such as a universal direct current motor.
It is a more specific object of the invention to provide a power control circuit which will provide a complete range of control voltages for smoothly operating the driven device from full "off" to full "on".
It is a further object of the invention to provide a power control circuit of the foregoing type which minimizes heat build-up and power loss both in the control circuit itself and in the driven device.